ITV was fined a record £5.7m by the media watchdog yesterday for "seriously and repeatedly misleading its audience" on 86 separate occasions over four years, causing viewers to waste £7.8m on worthless premium rate calls to some of its biggest hit entertainment shows.

Ofcom's findings detailed the full scope of what chairman Michael Grade has admitted was "a serious cultural failure" at Britain's biggest commercial broadcaster, with millions of viewers fooled into thinking they could join in with its shows at up to £1 a time.

The watchdog paints a picture of "omnipotent" senior producers wielding "unfettered power" and making sure junior staff who raised the issue were "firmly sat upon". They were preoccupied with bringing in large audiences while failing to understand they had "ceded editorial control" of elements of the show to viewers.

The regulator said the cases concerning Ant and Dec's Saturday Night Takeaway, in which competition winners were selected on the basis of where they lived or whether they would make good TV, were the most serious it had ever considered.

MPs immediately questioned why Grade had not fired any of his senior executives over the debacle.

In the case of Saturday Night Takeaway and other prime time shows, including Soapstar Superstar and Gameshow Marathon, senior producers repeatedly and intentionally misled millions of viewers.

ITV admitted to Ofcom that when more junior members of the Soapstar Superstar production team tried to raise the issue, they were "firmly sat upon" by senior producers. The judgment also revealed the partnership director and controller of ITV Interactive was informed there had been an "issue" with Soapstar Superstar but had not probed further.

Ant McPartlin and Declan Donnelly were both listed as executive producers of Saturday Night Takeaway, which was co-produced by their company, Gallowgate. When the revelations came to light, they said they were not aware of any malpractice and Grade tried to defend them by saying their titles were "vanity credits".

Ofcom said the repeated transgressions over a period of four years were made worse by the fact they occurred in primetime on shows with up to 9 million viewers, up to 1 million of them children.

The regulator said the fine, larger than had been expected, would have been higher still had the network not commissioned its own investigation from Deloitte, apologised, undertaken a wide-ranging overhaul of its procedures and promised to repay the £7.8m wasted by viewers.

ITV said yesterday just £10,074 worth of calls had been reclaimed by viewers, with the rest of the money donated to charity.

The fine is almost three times the previous record, when GMTV was fined £2m by Ofcom last September. The breakfast broadcaster, 75% owned by ITV, had persuaded viewers to waste up to £35m over a four year period on competitions they had no chance of winning.

Channel 4 has now scrapped premium rate phone-ins after it was fined £1.5m over similar incidents on Richard and Judy's You Say, We Pay competition and Deal or No Deal.

The revelations over premium rate phone lines have proved one of the main unexpected obstacles in Grade's path as he has attempted to revive the broadcaster's fortunes since arriving from the BBC in January last year. Last month, ITV revealed it had lost £58m in revenues as a result of the premium phone line scandals.

He was criticised by some for failing to sack anyone after earlier promising "zero tolerance" on viewer deception. But he claimed that starting a "witch-hunt" would discourage whistle blowers from coming forward and his strategy of apparent contrition appears to have saved ITV from an even bigger sanction.

Ed Richards, Ofcom's chief executive, said: "The industry can be in no doubt how seriously Ofcom takes the issue of audience trust." Ofcom will rule within weeks on the most high profile cases still on its books, concerning faked competitions on BBC shows including Comic Relief.

The Liberal Democrat media spokesman, Don Foster, said ITV "should count itself lucky" the fine was not bigger. "Questions must be asked about why someone in the BBC loses their job for misrepresenting the Queen, but no one at ITV goes for conning the public," he added.